Syntheses and characterization of copper complexes of the ligand (2-aminoethyl)bis(2-pyridylmethyl)amine (uns-penp) and derivatives

Article abstract of DOI:10.1039/b208740e

Synthesis and structural characterization of the copper(I) and copper(II) complexes of the tripodal tetradentate ligand (2-aminoethyl)bis(2-pyridylmethyl) amine (uns-penp) are reported as well as the reactivity of the copper(I) uns-penp complex towards dioxygen. Uns-penp can be easily modified and two copper(II) complexes of these derivatives have been structurally characterized. In the copper(II) complex of an amide derivative of uns-penp, [Cu(acetyl-uns-penp)Cl]ClO₄, it is evident that the amide nitrogen is no longer truly sp² hybridised, a consequence of a weak interaction with the copper(II) ion. The Royal Society of Chemistry 2003.

Full text of DOI:10.1039/b208740e

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Syntheses and characterization of copper complexes of the ligand
(2-aminoethyl)bis(2-pyridylmethyl)amine (uns-penp) and derivatives
Markus Schatz,a Michael Leibold,a Simon P. Foxon,†a Markus Weitzer,a
Frank W. Heinemann,a Frank Hampel,b Olaf Walterc and Siegfried Schindler*†a
a Institute of Inorganic Chemistry, University of Erlangen-Nürnberg, Egerlandstraße 1,
91058 Erlangen, Germany
b Institute of Organic Chemistry, University of Erlangen-Nürnberg, Henkestraße 42,
91054 Erlangen, Germany
c ITC-CPV, Forschungszentrum Karlsruhe GmbH, ITC-CPV, Postfach 3640, 76021 Karlsruhe,
Germany
Received 9th September 2002, Accepted 18th February 2003
First published as an Advance Article on the web 10th March 2003
Synthesis and structural characterization of the copper() and copper() complexes of the tripodal tetradentate
ligand (2-aminoethyl)bis(2-pyridylmethyl)amine (uns-penp) are reported as well as the reactivity of the copper()
uns-penp complex towards dioxygen. Uns-penp can be easily modified and two copper() complexes of these
derivatives have been structurally characterized. In the copper() complex of an amide derivative of uns-penp,
[Cu(acetyl-uns-penp)Cl]ClO4, it is evident that the amide nitrogen is no longer truly sp2 hybridised, a consequence
of a weak interaction with the copper() ion.
Introduction
Copper ions are found in the active sites of a large number of
proteins involved in redox processing of molecular oxygen.1–8
Low molecular weight model complexes for such enzymes have
been synthesized and their reactions with dioxygen investi-
gated.1,4,9–14 These model compounds not only provide better
understanding of the biological molecules but furthermore,
they assist in the development of new homogeneous catalysts
for selective oxidations under mild conditions.5,6,9,10,13–17
A
whole variety of copper dioxygen adducts can form when
copper() complexes are reacted with dioxygen.12,13 The course
of these reactions depends on temperature, ligand and sol-
vent.12,13 Therefore, it is of great interest to elucidate the factors
that govern the (reversible) binding and activation of dioxygen
with copper() complexes.
The first example of a structurally characterized copper per-
oxo complex was obtained by Karlin and coworkers from the
reaction of [Cu(tmpa)(CH3CN)]PF6 (tmpa = tris[(2-pyridyl)-
methyl]amine, Scheme 1) with O2 at low temperatures.18–20
A
detailed kinetic investigation of the reversible reaction of
[Cu(tmpa)(CH3CN)]ϩ with O2 was performed in propionitrile
which allowed the spectroscopic observation of a superoxo
complex prior to formation of the peroxo complex.21 Further-
more, the influence of sterical hindrance as well as different
donor atoms on the formation of the peroxo complexes was
studied.21–24 More recently we have investigated the effect of
chelate ring size on the properties of these complexes.25,26
To further elucidate the factors that influence the reactions of
dioxygen with Cu() complexes we have started to use tripodal
ligands derived from the parent compound tris(2-amino-
ethyl)amine (tren, Scheme 1).13 Tren is a versatile tetradentate
ligand, known to form trigonal bipyramidal complexes with
copper() ions in the solid state as well as in solution (similar to
copper() tmpa complexes).27–29 One of the axial coordination
sites around the copper() ion is occupied by the tertiary amine
nitrogen, the other by water or some other monodentate
ligand.13,28 In contrast to [Cu(tmpa)(CH3CN)]ϩ the copper()
Scheme 1
tren complex does not form a stable dioxygen adduct on O2
exposure.13,30 However, if the fully methylated form of tren,
tris(2-dimethylaminoethyl)amine (Me6tren, Scheme 1) is used
the formation of a copper superoxo and peroxo complex at low
temperatures is observed in a similar manner to tmpa.30,31
Therefore, we became interested in investigating how a ligand
incorporating both amine as well as pyridine arms would influ-
ence the reactivity of its copper() complexes towards dioxygen
(Scheme 1). Both the amines (2-aminoethyl)bis(2-pyridyl-
methyl)amine (uns-penp, Scheme 1) and bis[2-aminoethyl-
(2-pyridylmethyl)]amine (apme, Scheme 1) as well as some
† Present address: Institut für Anorganische und Analytische Chemie,
Justus-Liebig-Universität Gießen, Heinrich-Buff-Ring 58, 35392
Gießen, Germany. E-mail: siegfried.schindler@anorg.chemie.uni-
giessen.de; Fax: +49 641 9934140.
D a l t o n T r a n s . , 2 0 0 3 , 1 4 8 0 – 1 4 8 7
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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derivatives have been described in the literature, but have been
used only recently in a few cases for the syntheses of metal
complexes.32–42
The respective methylated forms of these ligands, Me2-uns-
penp and Me4-apme have not been reported prior to our
work.43,44 Herein we describe the syntheses, properties and reac-
tions of copper complexes of the ligand uns-penp and some of
its derivatives.
Results and discussion
The ligand uns-penp
Amines are versatile ligands in coordination chemistry because
they can be readily derivatised, thus allowing the introduction
of further functional groups. For example amines react with
aldehydes to form imines which can be subsequently reduced
to secondary or tertiary amines. With tren a whole series of
new ligands has been described where all three ‘arms’ have
been modified (only some examples are given in the refer-
ences).13,30,45–52 It is possible to chemically derivatise only one
‘arm’ of tren but often ligand mixtures are obtained.53,54 This
can be avoided if a tren derivative is used that has only one
modifiable amine group available. Therefore, a ligand such as
uns-penp (Scheme 1) has much potential for incorporating
further donor atoms or for the formation of dinucleating
ligands with various spacer groups.13
The synthesis of uns-penp as well as the copper() complex
of the protonated form uns-Hpenp were reported by Mandel
and co-workers.32,55 Unfortunately, the ligand uns-penp was
described as a “new ligand” ten years later by Matouzenko et
al. using a different synthetic route.33 A slightly modified syn-
thesis has been described recently by Girerd and coworkers as
well as by Rheingold, Bosnich and coworkers.37,56 A different
approach for the preparation of uns-penp was employed by
Nagano and co-workers.57 All authors have failed to acknow-
ledge the original reports by Mandel et al. These recent
synthetic routes described in the literature allow uns-penp to
be prepared in acceptable yields. The ligand can be routinely
purified by chromatography or by distillation.56,57
Fig. 1 ORTEP representation (50% thermal probability ellipsoids) of
the crystal structure of the cation of 1.
hydrochloride salt. At the beginning of our work we observed
that it was quite difficult to obtain uns-penp (as well as its
derivatives, see below) in a pure form because this amine had
the tendency to strongly bind one proton. Therefore our efforts
to synthesize 1 resulted first in the isolation of a copper()
compound with the protonated ligand (uns-Hpenp). The
complex [{Cu(uns-Hpenp)Cl}2Cl](ClO4)3 (2) was prepared by
reacting stoichiometric amounts of copper() salts and the
ligand in a mixture of methanol and water. 2 crystallizes in
the monoclinic space group C2/c with Z = 8; a summary of
crystal structure parameters and refinement results for this
compound is given in Table 1, selected bond distances and
angles in Table 2. An ORTEP plot of the cation of 2 is shown in
Fig. 2.
We have developed a facile two-step synthesis for the prepar-
ation of uns-penp, which is based upon a modified version of
the original synthesis described by Mandel et al. N-Acetyl-uns-
penp was synthesized in high yield by the in situ reductive alkyl-
ation of N-acetylethylenediamine with 2-pyridylcarbaldehyde
using NaBH(OAc)3.58 Our synthetic route obviates the need of
using the unpleasant chemical picolyl chloride and is one step
shorter than the more recently reported procedures for the
synthesis of uns-penp which employ N,N-bis(2-pyridylmethyl)-
amine.
Fig. 2 ORTEP representation (50% thermal probability ellipsoids) of
the crystal structure of the cation of 2.
Copper(II) complexes
[Cu(uns-penp)Cl]ClO4 (1). The copper() complex [Cu(uns-
penp)Cl]ClO4 (1) was prepared by reacting stoichiometric
amounts of Cu(ClO4)2, CuCl2 and uns-penp. Crystals suitable
for structural characterization were obtained and an ORTEP
plot of the cation of 1 is shown in Fig. 1 (crystallographic data
are reported in Tables 1 and 2).
The geometry around the copper() centre is best described
as trigonal bipyramidal with the chloride ion and the tertiary
nitrogen atom occupying the axial coordination sites, and the
pyridine nitrogen atoms together with the primary amine nitro-
gen atom in the equatorial positions. This is analogous to the
structures of the respective copper() complexes of tmpa
and Me6tren.30,59 In contrast, and as discussed previously, the
structure of [Cu(Me2-uns-penp)Cl]ClO4 is better described as
distorted square pyramidal.43
This complex crystallizes in a dimeric form where each
copper() ion is ligated by the two aromatic nitrogen atoms of
the pyridyl groups, the nitrogen atom of the tertiary amines and
a chloride ion. Furthermore the two copper() ions are bridged
by a chloride ion. The aliphatic primary amine arms of uns-
penp, which are protonated do not coordinate to the copper()
ions. The coordination geometry around the copper() centres
is best described as square pyramidal. Our crystal structure is
clearly different from the one reported earlier by Mandel and
Douglas which is monomeric and in which the coordination
geometry around the copper() centre is distorted trigonal bi-
pyramidal.55 Two pyridyl groups of uns-Hpenp are found to
occupy the axial coordination positions around the copper()
centre. The equatorial plane consists of the two chloride ions
and the tertiary amine nitrogen of the ligand. The reason for
obtaining different crystals most likely is a consequence of the
slightly different crystallisation conditions employed.
[{Cu(uns-Hpenp)Cl}2Cl](ClO4)3 (2). In the original work
on the synthesis of uns-penp the ligand was isolated as the
D a l t o n T r a n s . , 2 0 0 3 , 1 4 8 0 – 1 4 8 7
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Table 1
Complex
1
2
3
4
5
Molecular formula
Mr
Temperature/K
Radiation used (λ/Å)
Crystal system
Space group
a/Å
C14H18Cl2CuN4O4
440.77
298(2)
C14H19Cl3CuN4O6.50
517.22
200(2)
C16H20Cl2CuN4O5
482.80
200(2)
C42H57Cl6Cu2N8O24.50
1405.74
200(2)
C30H37Cl2N9O8Cu2
849.67
173(2)
Mo-Kα (0.71073)
Monoclinic
P21/n
9.101(4)
13.133(5)
15.199(5)
101.91(3)
1778(2)
Mo-Kα (0.71073)
Monoclinic
C2/c
14.3876(13)
20.6906(18)
14.8827(13)
108.8280(10)
4193.3(6)
8
Mo-Kα (0.71073)
Monoclinic
P21/n
12.4114(11)
11.7813(10)
13.7910(12)
105.5720(10)
1942.5(3)
4
Mo-Kα (0.71073)
Monoclinic
C2
24.686(2)
10.4701(8)
24.507(2)
114.146(2)
5780.0(8)
4
Mo-Kα (0.71073)
Monoclinic
C2/c
12.6284(2)
16.8880(2)
34.5479(5)
92.8550(6)
7358.83(18)
8
b/Å
c/Å
β/Њ
V/Å3
Z
4
Dc/g cmϪ3
1.647
1.556
1.639
1.465
1.651
1.436
1.615
1.100
1.534
1.361
µ/mmϪ1
Crystal size/mm
Reflections measured
Unique reflections, Rint
Refined parameters
R(F, F 2 > 2σ)
Rw(F 2, all data)
Abs. structure parameter76
Max./min. el. dens./e ÅϪ3
0.58 × 0.45 × 0.20
4797
3898, 0.0332
317
0.0418
0.0917
0.35 × 0.25 × 0.20
22039
5135, 0.0265
311
0.0508
0.1802
0.3 × 0.3 × 0.04
20630
4787, 0.0796
265
0.0377
0.0829
0.5 × 0.3 × 0.6
30211
13756, 0.0431
759
0.0477
0.1333
0.30 × 0.30 × 0.20
11855
6935, 0.0222
460
0.0458
0.1523
0.045(10)
0.616, Ϫ0.671
0.420, Ϫ0.376
1.555, Ϫ0.726
0.356, Ϫ0.325
0.707, Ϫ0.438
Table 2 Selected bond lengths (Å) and angles (Њ)
1
3
N(7)–Cu(2)–N(6)
N(7)–Cu(2)–N(5)
N(6)–Cu(2)–N(5)
N(7)–Cu(2)–Cl(1)
N(6)–Cu(2)–Cl(1)
N(5)–Cu(2)–Cl(1)
N(7)–Cu(2)–O(3)
N(6)–Cu(2)–O(3)
N(5)–Cu(2)–O(3)
Cl(1)–Cu(2)–O(3)
Cu(2)–Cl(1)–Cu(1)
165.0(2)
82.3(2)
83.3(2)
97.2(1)
96.0(1)
168.7(1)
94.2(2)
93.1(2)
101.6(2)
89.8(1)
96.1(1)
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–N(4)
Cu(1)–Cl(1)
2.129(3)
2.054(3)
2.047(3)
2.059(4)
2.241(2)
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–Cl(1)
2.031(2)
1.985(3)
1.979(2)
2.235(1)
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(3)
N(1)–Cu(1)–Cl(1)
N(2)–Cu(1)–N(3)
N(2)–Cu(1)–Cl(1)
N(3)–Cu(1)–Cl(1)
83.8(1)
82.5(1)
179.0(1)
165.9(1)
97.1(1)
96.6(1)
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(3)
N(1)–Cu(1)–N(4)
N(1)–Cu(1)–Cl(1)
N(2)–Cu(1)–N(3)
N(2)–Cu(1)–N(4)
N(2)–Cu(1)–Cl(1)
N(3)–Cu(1)–N(4)
N(3)–Cu(1)–Cl(1)
N(4)–Cu(1)–Cl(1)
118.4(2)
80.2(2)
108.9(2)
97.6(1)
81.8(2)
127.0(2)
99.3(1)
84.1(2)
177.7(1)
96.9(2)
5
4
Cu(1)–N(1)
Cu(1)–N(20)
Cu(2)–N(2)
Cu(1)–N(10)
Cu(1)–N(7)
Cu(2)–N(3)
Cu(2)–N(30)
Cu(2)–N(40)
2.282(3)
2.030(3)
2.319(3)
2.058(3)
2.017(3)
1.999(3)
2.031(3)
2.047(3)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–N(1)
Cu(1)–Cl(2)
Cu(1)–Cl(1)
Cu(2)–N(7)
Cu(2)–N(6)
Cu(2)–N(5)
Cu(2)–Cl(1)
Cu(2)–O(3)
1.981(3)
1.984(3)
2.056(3)
2.255(1)
2.688(1)
1.967(3)
1.983(4)
2.066(3)
2.293(1)
2.357(3)
2
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–Cl(1)
Cu(1)–Cl(2)
Cu(1)–Cl(2)*
2.068(3)
1.970(3)
1.972(3)
2.288(1)
2.683(1)
2.683(1)
N(1)–Cu(1)–N(7)
N(1)–Cu(1)–N(10)
N(1)–Cu(1)–N(20)
N(7)–Cu(1)–N(10)
N(7)–Cu(1)–N(20)
N(10)–Cu(1)–N(20)
N(2)–Cu(2)–N(3)
N(2)–Cu(2)–N(30)
N(2)–Cu(2)–N(40)
N(3)–Cu(2)–N(30)
N(3)–Cu(2)–N(40)
N(30)–Cu(2)–N(40)
132.6(2)
80.5(2)
79.9(2)
113.6(2)
123.1(2)
117.5(2)
132.2(2)
79.4(2)
N(2)–Cu(1)–N(3)
N(2)–Cu(1)–N(1)
N(3)–Cu(1)–N(1)
N(2)–Cu(1)–Cl(2)
N(3)–Cu(1)–Cl(2)
N(1)–Cu(1)–Cl(2)
N(2)–Cu(1)–Cl(1)
N(3)–Cu(1)–Cl(1)
N(1)–Cu(1)–Cl(1)
Cl(2)–Cu(1)–Cl(1)
165.0(2)
82.9(2)
82.2(2)
97.3(1)
97.3(1)
173.7(1)
87.9(1)
93.9(1)
90.1(1)
96.2(1)
N(2)–Cu(1)–N(1)
N(2)–Cu(1)–N(3)
N(1)–Cu(1)–N(3)
N(1)–Cu(1)–Cl(1)
N(2)–Cu(1)–Cl(1)
N(3)–Cu(1)–Cl(1)
N(1)–Cu(1)–Cl(2)
N(2)–Cu(1)–Cl(2)
N(3)–Cu(1)–Cl(2)
Cl1(1)–Cu(1)–Cl(2)
83.0(2)
163.6(2)
82.9(2)
166.6(1)
96.0(1)
95.9(1)
104.4(1)
95.9(1)
95.6(1)
89.0(1)
79.1(2)
124.6(2)
122.2(2)
105.8(2)
[Cu(acetyl-uns-penp)Cl]ClO4 (3). Following the original
synthetic report, the acetyl-protected form of the ligand was
isolated as an intermediate product during the synthetic pro-
cedure.32 Therefore, we decided to synthesize and characterize a
copper() complex of this ligand, in order to ascertain whether
or not the amide nitrogen was coordinated to the copper()
centre. Mixing stoichiometric amounts of Cu(ClO4)2, CuCl2
and acetyl-uns-penp in aqueous methanol lead to the formation
of [Cu(acetyl-uns-penp)Cl]ClO4 (3). This complex crystallizes
in the monoclinic space group P21/n with Z = 4 and an ORTEP
plot of the cation of 3 is presented in Fig. 3 (a summary of
crystal parameters, selected bond distances and angles are given
in Tables 1 and 2).
The copper() centre is four coordinate, being ligated by the
two pyridine groups, the aliphatic tertiary amine nitrogen and a
chloride ion. The amide nitrogen atom does not coordinate to
the copper() centre [Cu(1)–N(4) 2.640(2) Å]. The amide nitro-
gen atom N(4) does however form a weak electrostatic inter-
action with the copper() centre. Analysis of the torsional
angles around the amide unit according to Winkler and Danitz
allows the degree of “pyramidalization” at the amide nitrogen
atom to be calculated.60,61 From this analysis it is evident that
D a l t o n T r a n s . , 2 0 0 3 , 1 4 8 0 – 1 4 8 7
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Fig. 4 ORTEP representation (50% thermal probability ellipsoids) of
the crystal structure of the cation of 4.
The structure is dimeric in the solid-state. Each copper()
centre is ligated by the two nitrogen atoms from the bis(pyridyl-
methyl)amine-moiety and the tertiary amine nitrogen (for
Cu(1); N(2) and N(3), and N(1), respectively). A chloride ion
(Cl(2) and Cl(1)) completes the respective basal planes around
each copper() centre. The coordination geometry around each
copper() centre is best described as square pyramidal. The
apical coordination position of Cu(1) is occupied by Cl(1),
and for Cu(2) a water molecule O(3) occupies the apical site.
Furthermore Cl(1) bridges the two copper() centres. Both the
secondary amine nitrogens N(4) and N(8) are protonated and
the two salicyl “arms” retain their phenol groups and are not
coordinated to the copper() centres.
Fig. 3 ORTEP representation (50% thermal probability ellipsoids) of
the crystal structure of the cation of 3.
the nitrogen atom N(4) is no longer truly sp2 hybridised. The
geometry around the copper() centre is distorted square
planar, as is evident from the following angles [N(1)–Cu(1)–
Cl(1) 179.01(7)Њ, N(2)–Cu(1)–N(3) 165.93(11)Њ]. In contrast, no
interaction between the copper() ion and the amide nitrogen
was reported (distance 2.827(4) Å) for the copper() complex of
the N-benzyl-uns-penp analogue, structurally characterized by
Nishida and coworkers.62
N-acetyl-uns-penp resembles the amino acid derivatised
bispicolylamine ligands prepared by Alsfasser and co-
workers63,64 However, in such metal complexes the amide oxy-
gen atom is found to be coordinated to the respective metal
centres (Cu(), Ni(), Zn()). The motivation for such research
is in preparing small molecule model complexes that mimic
the metal-coordination by peptide frameworks, which is
hypothesised as playing a crucial role in key biological
metal-induced pathogenic events.65
Efforts to obtain crystals of copper complexes with the
unprotonated salicyl-uns-penp are in progress but all attempts
so far have been unsuccessful.
Copper(I) complexes
Only the copper() complex of uns-penp could be prepared by
mixing stoichiometric amounts of [Cu(CH3CN)4]ClO4 with the
amine in acetonitrile under argon. The syntheses of copper()
complexes of the other ligands were unsuccessful. Crystals of
the copper() complex of uns-penp suitable for structural char-
acterization formed by slow diffusion of diethyl ether into an
acetonitrile solution in the glove box. It is worth noting that we
were able to obtain crystals of this complex because all our
attempts to isolate crystals of the copper() tren complex in an
analogous manner have been to date unsuccessful (most of the
time disproportionation was observed30).
[(H2O)(salicyl-uns-Hpenp)Cu(Cl)Cu(salicyl-uns-Hpenp)Cl]-
(ClO4)4 (4). As discussed above, uns-penp has the advantage
to readily allow the introduction of further functional
groups starting with a Schiff base condensation. This approach
was demonstrated successfully by Girerd and coworkers who
employed salicylaldehyde and subsequently reduced the imine
to the according amine.37 However, during the attempted
synthesis of the manganese complex of salicyl-uns-penp
(Scheme 1) it turned out that dehydrogenation of the ligand
had occurred and only crystals of the manganese imine com-
plex were obtained. This observation is quite similar to the
formation of an imine copper() complex (here subse-
quent hydrolysis occurred additionally) during the reaction
of [Cu(Bz3tren)Cl](ClO4) (Bz3tren = tris(2-benzylaminoethyl)-
amine) or [Cu(Bz3tren)H2O](ClO4)2 with dioxygen described
recently by us.52
An ORTEP plot of the cation of [Cu2(uns-penp)2](ClO4)2 (5)
is shown in Fig. 5 (a summary of crystallographic data and
refinement parameters can be found in Table 1; selected bond
lengths and angles are reported in Table 2).
Related to salicyl-uns-penp is the ligand pyridyl-uns-penp
that can be prepared in a similar manner. However, pyridyl-uns-
penp and its derivatives have been synthesized in a different way
and the ligands are abbreviated in the literature as trispic or
tpen.39–42 They proved to be quite useful in the understanding of
the formation of iron peroxo species.39–42
At the beginning of our own work we were unaware of the
difficulties to remove the strongly bound proton in this kind of
amine ligands (see discussion above). Therefore, the product
from the synthesis of salicyl-uns-penp was mainly salicyl-uns-
penpؒxHCl. The reaction of this protonated amine with
Cu(ClO4)2 in a mixture of methanol and water lead to the
formation of [(H2O)(salicyl-uns-Hpenp)Cu(Cl)Cu(salicyl-uns-
Hpenp)Cl](ClO4)4 (4). It crystallizes with 5.5 water molecules in
the asymmetric unit. The positions of the hydrogen atoms of
the non-coordinated water molecules could not be located and
therefore were omitted for the refinement. An ORTEP plot of
the cation of 4 is shown in Fig. 4 (a summary of crystal param-
eters, selected bond distances and angles are given in Tables 1
and 2).
Fig. 5 ORTEP representation (50% thermal probability ellipsoids) of
the crystal structure of the cation of 5.
Fig. 5 illustrates that the complex crystallizes as a four-
coordinate copper() dimer which incorporates two uns-penp
ligands with the two copper() ions separated by 3.933(3) Å.
Each copper() ion is ligated by two pyridyl moieties and the
tertiary amine group from one uns-penp ligand unit. The fourth
D a l t o n T r a n s . , 2 0 0 3 , 1 4 8 0 – 1 4 8 7
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donor, the nitrogen atom of the primary amine is supplied by
the other ligand unit. The geometry around each copper() ion
is best described as distorted tetrahedral. The tertiary amine
nitrogen atoms [N(1) or N(2)] bind more weakly to the
copper() ion [Cu(1)–N(1) 2.232(3) Å] compared to the other
three nitrogen atoms [Cu(1)–N = 2.017(3)–2.058(3) Å]. There is
another example of a similar dimeric copper() complex
([Cu2(BPIA)2](CF3SO3)2, (BPIA = bis((2-pyridyl)methyl)((1-
methylimidazol-2-yl)methyl)amine)) described in the literature
that also shows coordination of two tripodal ligands.23
insert in Fig. 6. Under the conditions employed this complex is
completely formed in about 40 s but decomposition is observed
shortly afterwards.
As described earlier the solvent can have a dramatic effect on
the reaction of a copper() complex with dioxygen.30,66 Here we
observe a large rate increase when the oxidation of 5 is analyzed
in acetone instead of propionitrile. In Fig. 7 time resolved
spectra for this reaction at Ϫ89.9 ЊC in acetone are shown.
The observed dimeric structure of the above complex is dif-
ferent from that seen for other copper() complexes with similar
tripodal tetradentate ligands, such as tmpa or Me6tren, where
only mononuclear complexes were isolated and characterized.
This is furthermore demonstrated by the crystal structure of the
related copper() complex of Me2-uns-penp.43 In contrast to 5,
this complex is mononuclear and the coordination geometry
around the copper() ion is best described as slightly distorted
trigonal bipyramidal, with an additional acetonitrile solvent
molecule occupying an axial position.
Reactivity of 5 towards dioxygen
The reaction of dioxygen with copper() complexes with tri-
podal tetradentate ligands has been studied extensively by us
and others.1,6,13,21,25,30,31,52,66–68 It was noticed during this work
that the presence of protons destabilized to a large extent the
“dioxygen adduct” complexes that formed as intermediates dur-
ing the oxidation reaction at low temperatures.30 Protons could
come from the solvent (e.g. methanol) or from the ligand itself.
When tren is used as a tripodal ligand, only a very unstable
“dioxygen adduct”, most likely a superoxo complex, was
observed during the reaction at low temperatures.13 In contrast
single alkylation of each ‘amine arm’ leading to Bz3tren allowed
the spectroscopic observation of a peroxo complex at low tem-
peratures.52 Full alkylation of each ‘amine arm’ leading to
Me6tren further increased the stability of the formed “dioxygen
adduct” complexes.13,30,31,68 To gain further insight into the
reaction of dioxygen with copper() complexes with “proton
rich” ligands we used the ligand uns-penp which in contrast to
tren has only one primary ‘amine arm’.
Propionitrile or acetone solutions of 5 (formed in situ by
mixing stoichiometric amounts of copper() salt and uns-penp)
were reacted with dioxygen at low temperatures in a stopped-
flow unit equipped with a diode array detector. A typical
example of time resolved spectra for this reaction at Ϫ89.9 ЊC
in propionitrile is shown in Fig. 6. During the mixing time an
absorbance band at 410 nm has already formed that decreases
while another band is formed with an absorbance maximum at
486 nm. An absorbance vs. time trace at 486 nm is shown as an
Fig. 7 Time resolved spectra of the reaction of 5 with dioxygen in
acetone at Ϫ89.9 ЊC ([complex] = 0.4 mM, [O2] = 5.1 mM, total time =
0.77 s). Only every fifth spectrum is shown.
Here the absorbance maxima are clearly shifted to a large
extent to longer wavelengths. The band at 426 nm decreases
while the band at 535 nm increases. The insert shows that the
reaction under these conditions is complete in less than 0.1 s in
contrast to the 40 s that was observed in propionitrile.
The comparison of the UV-vis spectra with the spectral
features of known “copper dioxygen” complexes indicates that
superoxo (high energy band) and peroxo (low energy band)
copper complexes are formed.21,23,30,68 This is in accord with the
behaviour observed in the two different solvents: in propio-
nitrile the solvent is competing with dioxygen as a ligand and
therefore equilibria are pushed to the side of the reactants in
propionitrile while in acetone the formation of the peroxo com-
plex is favoured. This is documented by the higher rates in acet-
one discussed above and also by a comparison of the relative
absorptivities of the formed species: lower absorptivity for the
superoxo complex in contrast to the higher absorptivity of the
peroxo complex (in acetone). However, due to the decom-
position reactions described above, a clear assignment of these
species by Resonance Raman measurements was not possible.
Furthermore, it is not clear what causes the large shift of the
absorbance maxima going from propionitrile to acetone, an
experimental result that was not observed for the corresponding
copper complexes of tmpa and Me6tren. An attempted kinetic
analysis showed that the data could not be fitted to the same
mechanistic model that was used for the copper complexes of
tmpa and Me6tren.68 Additional reactions take place and the
reaction behaviour is more complex, probably a consequence of
the dimeric nature of 5.
Although the reaction mechanism and the detailed nature of
the superoxo and peroxo complexes could not be determined
completely, a comparison with the oxidation of the copper()
complex with the ligand Me2-uns-penp, the methylated form of
uns-penp, is quite useful. As described earlier [Cu(Me2-uns-
penp)(CH3CN)]ClO4 reacts with dioxygen to form a peroxo
complex that is persistent even at room temperature for a short
time.43 Therefore it is most likely that again the presence of the
protons from the primary amine group in uns-penp destabilize
the formed peroxo complex.
Fig. 6 Time resolved spectra of the reaction of 5 with dioxygen in
propionitrile at Ϫ89.9 ЊC ([complex] = 0.4 mM, [O2] = 4.4 mM, total
time = 46 s). Only every seventh spectrum is shown.
D a l t o n T r a n s . , 2 0 0 3 , 1 4 8 0 – 1 4 8 7
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py-H), 7.69 (br, 1H, NHCOCH3), 7.64 (dt, 2H, py-H), 7.36
Conclusion
(d, 2H, py-H), 7.17 (m, 2H, py-H), 3.89 (s, 4H, pyCH2), 3.32 (q,
2H, CH2NHCOMe), 2.75 (t, 2H, NCH2), 2.03 (s, 3H,
NHCOCH3). 13C NMR (CDCl , 75 MHz): δ 170.0 (C᎐O),
Uns-penp is an attractive ligand in that it can be readily syn-
thesized and a variety of substituted functionalised derivatives
are amenable. Due to such versatility, research groups now have
started to employ uns-penp and its derivatized ligands in many
diverse areas of bioinorganic chemistry. In this regard, we have
modified an old synthesis for the facile preparation of the start-
ing ligand uns-penp and its derivatives and have described some
copper()/() complexes thereof.
The copper() complex of N-acetyl-uns-penp shows a weak
copper()–amide nitrogen interaction as is evident from an
analysis of the torsional angles around the amide unit.
Stopped-flow measurements of the reaction of the copper()
complex of uns-penp with dioxygen at low temperatures,
allowed the spectroscopic detection of a copper superoxo and
peroxo species. The corresponding copper() complex of
the methylated version of uns-penp (Me2-uns-penp) supports
a more persistent copper peroxo complex, most likely a
consequence of the absence of available protons in the ligand.
᎐
3
159.2, 149.1, 136.5, 123.2, 122.2, 59.5, 52.5, 37.7, 23.4 (CH3). IR
(KBr disc)/cmϪ1: 3249 (m), 3208 (m), 3055 (m), 2982 (m), 2940
(m), 2894 (m), 2821 (m), 2362 (m), 1712 (m), 1672 (s), 1591 (m),
1564 (s), 1473 (m), 1433 (m), 1396 (m), 1370 (m), 1284 (m), 1244
(m), 1152 (m), 1130 (w), 1111 (m), 1086 (w), 1046 (m), 984 (m),
937 (w), 889 (w), 764 (s), 671 (w), 600 (w), 523 (w), 470 (w), 456
(w), 408 (w).
Step 2: Removal of the acetyl protecting group. N-Acetyl-uns-
penp (2.20 g, 7.74 mmol) was dissolved in 5 M aqueous hydro-
chloric acid (50 mL). The yellow solution was heated at reflux
for 24 h. The solution was allowed to cool, and made basic (pH
10) by the careful addition of NaOH. An orange oil separated.
The mixture was extracted with CH2Cl2 (3 × 50 mL). The
organic fractions were combined and dried over anhydrous
MgSO4. The solution was filtered and concentrated in vacuo.
The oil was then triturated with diethyl ether (60 mL). The
yellow solution was decanted, and the solvent removed in vacuo
to give the product as a pale yellow–orange oil (1.3 g, 5.36
Experimental
1
General remarks
mmol, 70%). Overall yield 62% (two steps). H NMR (CDCl3,
300 MHz): δ 8.45 (d, 2H, py-H), 7.58 (dt, 2H, py-H), 7.42 (d,
2H, py-H), 7.08 (m, 2H, py-H), 3.77 (s, 4H, pyCH2), 2.71 (t, 2H,
CH2NH2), 2.59 (t, 2H, CH2CH2NH2), 1.77 (s, 2H, NH2). 13C
NMR (CDCl3, 75 MHz): δ 160.0, 149.0, 136.6, 123.3, 122.3,
61.0, 57.7, 40.0. IR (neat oil, NaCl disc)/cmϪ1: 3687 (w), 3685
(m), 3649 (m), 3362 (s, br), 3290 (s, br), 3059 (s), 3010 (s), 2935
(s, br), 2825 (s), 2491 (m), 2195 (m), 1986 (m), 1917 (m), 1891
(m), 1770 (m), 1650 (s), 1590 (s), 1471 (s), 1436 (s), 1364 (s),
1307 (s), 1252 (s), 1147 (s), 1125 (s), 1090 (s), 1047 (s), 991 (s),
896 (s), 844 (s), 763 (s), 618 (s), 557 (s), 527 (m).
Reagents and solvents used were of commercially available
reagent grade quality. The purification of acetonitrile and pro-
pionitrile was done according to literature procedures:69 the
nitriles were stirred first over K2CO3 and then over P2O5 for
24 h, and distilled first from P2O5 and then K2CO3. Acetone was
distilled from dried B2O3. Diethyl ether was refluxed prior to
distillation over sodium metal until added benzophenone
showed a blue colour. [Cu(CH3CN)4]ϩ salts were synthesized
and characterized according to literature methods.70,71 Prepar-
ation and handling of air-sensitive compounds was carried out
in a glove-box filled with argon (Braun, Garching, Germany;
Salicyl-uns-penp. To a solution of uns-penp (0.5 g, 2.1 mmol)
in methanol (20 mL) salicylaldehyde (0.26 g, 2.1 mmol) was
added. The resulting Schiff base was reduced in situ with
NaBH4 (0.14 g, 3.6 mmol) which was added in small portions
and the solution was stirred for at least 2 h. The excess NaBH4
was destroyed by the careful addition of HCl, until the solution
had a pH value of 2. The methanol was removed in vacuo and
the residue was re-dissolved in a 5 M aqueous solution of
NaOH. The solution was extracted with CH2Cl2, the organic
solution was dried over Na2SO4 and the solvent was removed
in vacuo to yield salicyl-uns-penp. 1H NMR (CDCl3, 300 MHz):
δ 8.32 (m, 2H, py-H), 7.56–6.60 (m, 10 H, aromatic), 4.25 (s,
2H, HNCH2(2-hydroxybenzyl)), 3.73 (s, 4H, NCH2py), 3.05 (m,
4H, NCH2CH2NH).
1
water and dioxygen less than 1 ppm). The H NMR measure-
ments were recorded on a Bruker DXP 300 AVANCE spectro-
meter. Element analyses were performed on a Carlo Erba
Element Analyzer (type 1106) at the University of Erlangen-
Nürnberg. Time resolved spectra of the reactions of dioxygen
with copper() complexes were recorded on a modified Hi Tech
SF-3 L low temperature stopped-flow unit (HiTech, Salisbury,
UK) equipped with a J&M TIDAS 16–500 diode array spectro-
photometer (J&M, Aalen, Germany). Data fitting was per-
formed using the integrated J&M software Kinspec or the pro-
gram Specfit (Spectrum Software Associates, Chapel Hill,
USA). Dioxygen saturated solutions for the kinetic measure-
ments were obtained by bubbling dioxygen (Linde, Germany)
through the solvent for 20 min as described earlier.21
The ligands uns-penp and N-acetyl-uns-penp were syn-
thesized according to the published procedures discussed above
or as described below. The ligand uns-Hpenp was obtained fol-
lowing the original synthesis for uns-penp.32
[Cu(uns-penp)Cl]ClO4 (1). To a solution of uns-penp (0.5 g,
2.06 mmol) in methanol (15 mL) was added a solution of
Cu(ClO4)2ؒ6H2O (0.38 g, 1.03 mmol) and CuCl2ؒ2H2O (0.18 g,
1.03 mmol) in water (10 mL). The solution was stirred for
10 min and then filtered. Green crystals formed which were
suitable for X-ray analysis by slow evaporation of a methanol
solution. C14H18Cl2CuN4O4: calc. C 38.15, H 4.12, N 12.71;
found C 37.64, H 4.44, N 12.35%.
Synthesis of uns-penp
Step 1: Synthesis of N-Acetyl-uns-penp. N-Acetylethylene-
diamine (1.02 g, 10 mmol) and 2-pyridinecarbaldehyde (2.14 g,
20 mmol) were placed in 1,2-dichloroethane. NaBH(OAc)3
(6.0 g, 28.3 mmol) was added and the cloudy solution was
stirred at room temperature under nitrogen for 3 h. The reac-
tion was quenched by the addition of a 2 M aqueous solution
of NaOH (100 mL). The organic layer was separated and the
aqueous layer was extracted with CH2Cl2 (2 × 100 mL). The
organic fractions were combined and washed with a saturated
aqueous solution of NaCl (100 mL). The organic fraction was
dried over anhydrous Na2SO4. Filtration and removal of the
solvent in vacuo yielded the title compound as a cream coloured
solid (2.55 g, 8.97 mmol, 89%). Rf = 0.51 (EtOAc–MeOH, 95:5).
[{Cu(uns-Hpenp)Cl}2Cl](ClO4)3 (2). To a solution of H-uns-
penp (0.54 g, 2.2 mmol) in methanol (15 mL) was added a
solution of Cu(ClO4)2ؒ6H2O (0.42 g, 1.1 mmol) and CuCl2ؒ
2H2O (0.19 g, 1.1 mmol) in water (10 ml). The solution was
stirred for 10 min and filtered. Green crystals formed which
were suitable for X-ray analysis by slow evaporation of a
methanol solution.
[Cu(acetyl-uns-penp)Cl]ClO4 (3). To a solution of acetyl-uns-
penp (0.6 g, 2.1 mmol) in methanol (15 ml) was added a solu-
tion of Cu(ClO4)2ؒ6H2O (0.46 g, 1.1 mmol) and CuCl2ؒ2H2O
1
Mp 104–106 ЊC. H NMR (CDCl3, 300 MHz): δ 8.56 (d, 2H,
D a l t o n T r a n s . , 2 0 0 3 , 1 4 8 0 – 1 4 8 7
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6 Bioinorganic Chemistry of Copper, ed. K. Karlin and Z. Tyeklar,
Chapman & Hall, New York, 1993.
7 H. Decker, R. Dillinger and F. Tuczek, Angew. Chem., 2000, 112,
1656.
8 E. Solomon, Angew. Chem., Int. Ed., 2002, 40, 4570.
9 E. Spodine and J. Manzur, Coord. Chem. Rev., 1992, 119, 171.
10 H. C. Liang, M. Dahan and K. D. Karlin, Curr. Opin. Chem. Biol.,
1999, 3, 168.
11 K. D. Karlin, Science, 1993, 261, 701.
12 A. G. Blackman and W. B. Tolman, Struct. Bonding (Berlin), 1999,
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13 S. Schindler, Eur. J. Inorg. Chem., 2000, 2311.
14 L. Que, Jr. and W. B. Tolman, Angew. Chem., Int. Ed., 2002, 41,
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15 Bioinorganic Catalysis, ed. J. Reedijk and E. Bouwman, Marcel
Dekker, New York–Basel, 2nd edn., 1999.
(0.21 g, 1.1 mmol) in water (10 ml). The solution was stirred for
10 min and filtered. Blue crystals formed which were suitable
for X-ray analysis by slow evaporation of a methanol solution.
C16H20Cl2CuN4O5: calc. C 39.80, H 4.18, N 11.60; found C
39.56, H 4.38, N 11.39%.
[(H2O)(salicyl-uns-Hpenp)Cu(Cl)Cu(salicyl-uns-Hpenp)Cl]-
(ClO4)4 (4). To a solution of salicyl-uns-penp (0.17 g, 0.48
mmol) in methanol (15 ml) was added a solution of Cu(ClO4)2ؒ
6H2O (0.184 g, 0.49 mmol) in water (10 mL). The solution was
stirred for 10 min and filtered. Blue crystals formed which were
suitable for X-ray analysis by slow evaporation of a methanol
solution.
16 Y. Wang, J. DuBois, B. Hedman, K. Hodgson and T. D. P. Stack,
Science, 1998, 279, 537.
17 P. Chaudhuri, M. Hess, T. Weyermüller and K. Wieghardt,
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J. Zubieta, J. Am. Chem. Soc., 1988, 110, 3690.
19 Z. Tyeklár, R. R. Jakobson, N. Wei, N. N. Murthy, J. Zubieta and
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20 B. Lim and R. Holm, Inorg. Chem., 1998, 37, 4898.
21 K. D. Karlin, N. Wei, B. Jung, S. Kaderli, P. Niklaus and
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22 T. N. Sorrell and D. L. Jameson, Inorg. Chem., 1982, 21, 1014.
23 N. Wei, N. Murthy, Z. Tyeklar and K. Karlin, Inorg. Chem., 1994,
33, 1177.
[Cu2(uns-penp)2](ClO4)2 (5). [Cu(CH3CN)4]ClO4 (0.23 g, 0.7
mmol) was added with stirring to a solution of uns-penp
(0.17 g, 0.7 mmol) in a small amount of acetonitrile in a glove
box. Diethyl ether was added to the orange solution until a
precipitate was observed to develop. The solution was filtered
through a medium porosity frit, and the solid was washed with
diethyl ether. The yellow powder was dissolved in a small
amount of acetonitrile. Yellow crystals suitable for X-ray char-
acterization were obtained by diffusion of diethyl ether into this
solution.
X-Ray crystallographic study. Single crystals were coated
with polyfluoropolyalkylether oil and mounted on a glass fiber
or sealed in a glass capillary. Data for [Cu(uns-penp)Cl]ClO4
were collected on a Nicolet R3m/V diffractometer at 298(2) K
and for ([Cu2(uns-penp)2](ClO4)2, on a Nonius Kappa diffract-
ometer with a CCD array detector at 173(2) K (graphite-mono-
chromator). Lorentz, polarization, and empirical absorption
corrections were applied. The structures were solved by direct
methods and refined on F 2 using full-matrix least-squares tech-
niques.72 Non-hydrogen atoms were refined with anisotropic
thermal parameters. Intensity data for all other complexes were
collected on a Siemens SMART 1000 CCD-Diffractometer
with graphite monochromated Mo-Kα radiation (λ = 0.71073
Å). The exposure time was 10 s per frame collected with the
ω-scan technique (∆ω = 0.3Њ). The collected reflections were
corrected for Lorentz, polarization and absorption effects. The
structures were solved by direct methods and refined by full
matrix least squares methods on F 2.73–75
24 N. Wei, N. N. Murthy, Q. Chen, J. Zubieta and K. D. Karlin,
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25 M. Schatz, M. Becker, F. Thaler, F. Hampel, S. Schindler, R. R.
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27 P. C. Jain and E. C. Lingafelter, J. Am. Chem. Soc., 1967, 89, 6131.
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CCDC reference numbers 186857 ([Cu(uns-penp)Cl]ClO4
(1)), 186855 ([{Cu(uns-Hpenp)Cl}2Cl](ClO4)3 (2)), 186854
([Cu(acetyl-uns-penp)Cl]ClO4 (3)), 186856 ([(H2O)(salicyl-uns-
34 H. Adams, N. A. Bailey, W. D. Carlisle, D. E. Fenton and G. Rossi,
J. Chem. Soc., Dalton Trans., 1990, 1271.
Hpenp)Cu(Cl)Cu(salicyl-uns-Hpenp)Cl](ClO4)4
186634 ([Cu2(uns-penp)2](ClO4)2 (5)).
See http://www.rsc.org/suppdata/dt/b2/b208740e/ for crystal-
lographic data in CIF or other electronic format.
(4))
and
35 H. Adams, M. R. J. Elsegood, D. E. Fenton, S. L. Heath and
S. J. Ryan, J. Chem. Soc., Dalton Trans., 1999, 2031.
36 T.-H. Cheng, Y.-M. Wang, W.-T. Lee and G.-C. Liu, Polyhedron,
2000, 19, 2027.
37 O. Horner, E. Anxolabehere-Mallart, M.-F. Charlot, L. Tchertanov,
J. Guilhem, T. A. Mattioli, A. Boussac and J.-J. Girerd,
Inorg. Chem., 1999, 38, 1222.
38 O. Horner, M.-F. Charlot, A. Boussac, E. Anxolabehere-Mallart,
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1998, 721.
39 A. J. Simaan, F. Banse, P. Mialane, A. Boussac, S. Un, T. Kargar-
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40 A. J. Simaan, S. Döpner, F. Banse, S. Bourcier, G. Bouchoux,
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Acknowledgments
The authors gratefully acknowledge financial support from
the DFG. Simon P. Foxon acknowledges a scholarship from
the DAAD. Furthermore, they thank Prof. Rudi van Eldik
(University of Erlangen-Nürnberg) for his support of this
work.
41 I. Bernal, M. Jensen, K. B. Jensen, C. J. McKenzie, H. Toftlund and
J. P. Tuchagues, J. Chem. Soc., Dalton Trans., 1995, 3667.
42 A. Hazell, C. J. McKenzie, L. P. Nielson and S. Schindler, J. Chem.
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43 M. Weitzer, M. Schatz, F. Hampel, F. W. Heinemann and
S. Schindler, J. Chem. Soc., Dalton Trans., 2002, 686.
44 M. Weitzer, S. P. Foxon, F. Hampel and S. Schindler, work in
progress.
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